The velocity or speed ultrasonic pulses in gas can vary with temperature, humidity and type of gas. In the prior art, it is common practice to place a fixed reflector or reference target a short distance from an ultrasonic transducer so that in response to each transmitted pulse of ultrasonic vibrations two echoes will be received, the first from the reference target reflector which is at a fixed distance from the transducer, and the second from a surface being measured. For example, in a appartus for measuring the level of contents in a storage tank or container, the content surface is used to produce a second echo or reflection received at the transducer. An example of this type system is disclosed in Soltz U.S. Pat. No. 4,578,997 wherein a transducer disposed at a position above a liquid surface has a reference point reflector located at a fixed distance therefrom and when the transducer is excited to emit pulses, they are intercepted by the liquid surface and by the reference reflector to produce both reference and liquid echo pulses that are returned to the transducer and detected thereby. The respective transit times of the reference echo and liquid echo are determined and the ratio between these transit times are computed to provide an output representing the liquid level which is thereby rendered independent of changes in the gaseous environment.
Berg et al. U.S. Pat. No. 3,554,014 discloses ultrasonic gauging system which uses a liquid bath as a temperature compensating means and provides a standard or reference which has a length which is close to the length of the work piece being measured so that changes in the length of the work place due to temperature can be minimized.
Lynch et al U.S. Pat. No. 3,482,647 discloses a device for measuring the speed of sound in a medium in which a reflecting target is spaced from a transducer by iron-nickel alloy rods. Temperature compensation is achieved by making the ratio of temperature coefficient C.sub.1 (for the rods) and C.sub.2 (for the reflecting target) equal to the ratio of L.sub.1 (the length of each rod) to L.sub.2 (the height of the target on its supporting plate). This utilizes the differential in expansion of the spacing rods and reflecting targets to achieve temperature compensation.
Massa U.S. Pat. No. 4,210,969 discloses ultrasonic ranging apparatus in which temperature compensation is effected by a small reflecting target which is spaced by an L-shaped rod a distance D.sub.1 from the transducer to produce signals which are used for calibration purposes. A similar temperature compensating system is shown in the piston position measurement device of Reuter et al U.S. Pat. No. 4,542,652, and Head et al U.S. Pat. No. 4,543,649.
Dumas U.S. Pat. No. 4,254,478 discloses ultrasonic measurement apparatus in which a velocity meter measures the speed of propagation in water. The velocity meter is not described in detail but its principle appears to be similar to Massa's.
In Skrgatic et al U.S. Pat. No. 4,388,708, an ultrasonic measurement system incorporates a temperature compensation scheme in which the reference paths and the unknown or variable path are contained in a common medium (light oil). Also see Willis U.S. Pat. No. 3,834,233, Bolton U.S. Pat. No. 3,184,969 and Dennis U.S. Pat. No. 4,248,087.
Parrish U.S. Pat. No. 4,448,202 and Soltz U.S. Pat. No. 4,470,299 disclose ultrasonic liquid level measuring systems in which a reference target or reflector is positioned at a predetermined position relative to the transducer to produce reference signal used to compute liquid level signals which are independent of changes in the gaseous environment.
In all these cases, where the single reference target is utilized, there are three basic problems. The first relates to the electrical delay time (.tau.) through the circuit, which is not known well enought to provide adequate compensation when the velocity of ultrasonic energy is to be determined. In the present invention such electrical delay time is effectively substracted out because two accurately spaced targets are used and the spacing is rendered substantially inmmune to temperature variations. The time interval between the return echoes or pulses is an accurate measure of the velocity because the distance is accurately known and the time between the two target pulses can be accurately measured electronically and electrical time delays (.tau.) cancel out. Secondly, the distance from the transducer face need not be accurately known since the only distance of concern is the spacing between the two targets which is rendered substantially immune to temperature variations. In the prior art, multiple reflections from other than the reference target can interfere with the measurement. In the present invention, reflections from the two reference targets and make it easier to discriminate unwanted reflections.
The object of the present invention is to provide an inexpensive, highly accurate, ultrasonic apparatus for measuring the speed of sound in a gaseous medium. According to the invention, a pair of reflectors are coaxially located and separated by a mounting member having a low coefficient of linear expansion so that the distance between the two reflecting surfaces is accurately maintained thereby eliminating the effect of temperature expansion and contraction on changing the distance between the two targets. A circuit is connected to the transducer for converting the ultrasonic energy reflections from the first and second ultrasonic reflecting surfaces to a signal corresponding to the speed of the ultrasonic energy between these two ultrasonic reflecting surfaces in the surrounding medium. Since the distance between the two reference targets is accurately known, the time of ultrasonic reflection or echoes from these two targets provide a highly accurate measure of the velocity of sound in the gaseous medium which then may be used to perform highly accurate ultrasonic measurements of distances for gauging of articles and the like. The transducer and reference targets are preferably contained in a cylindrical housing which has a diameter larger than the diameter of the largest ultrasonic reflector. Moreover, the mounting means is made of Invar metal which has a very low thermal expansion coefficient of linear expansion. It could also be a low expansion material ceramic material such as ULE.TM., CERUIT.TM.. The chamber also includes a port in one end thereof for admitting the gaseous medium to the chamber and a second port for removing the gaseous medium from the chamber so that the gaseous medium in which the desired speed measurement is to be made is maintained inside the chamber. A conventional temperature measuring device such as a thermister or other temperature sensitive element is incorporated in the chamber so that a measurement of the gas temperature may also be made. With the velocity and temperature measurements, the ratio of the average molecular weight to the specific-heat ratio of the gas can be computed. The velocity and temperature measurements are processed by a microprocessor and supplied to a utilization device.